When oil is present in subterranean rock formations such as sandstone or chalk, it can generally be exploited by drilling into the oil-bearing measures and allowing existing overpressures to force the oil up the borehole. This is known as primary removal. When the overpressure approaches depletion, it is customary to create an overpressure, for example by injecting water into the formations to flush out standing oil. This is known as secondary removal.
However, even after secondary removal, a great deal of oil remains in the formations; in the case of North Sea oil, this may represent up to 75% of the original oil present. Of this remaining oil probably more than half will be in the form of droplets and channels adhering to the rock formations that have been water-flooded and the remainder will be in pockets which are cut off from the outlets from the field. The present invention is concerned with the exploitation of the accessible but adhering oil remaining in the rock formations.
A number of enhanced oil recovery methods have been proposed, to address this objective. One approach is to combine pressure with a change in viscosity of the oil and/or water present. Thus, a diluent or CO2 or steam is added to the reservoir to reduce the viscosity of the oil, thereby allowing it to be freed. Alternatively, viscosity-increasing additions such as polymers may be added to the injection water so that the oil is preferentially dislodged. However, the application of CO2 is disadvantageous due to scale formation, the use of steam is only effective in shallow reservoirs of low temperature while the other additives are very costly.
Another approach is to alter the surface tension and capillary forces so that the water under pressure is more accessible to the pores and channels. This may be achieved by alkaline flooding or by means of surfactants. However, these approaches also tend to be costly.
Another approach is in situ combustion. This entails pumping air or oxygen into the formation and igniting the gas/oil present. In theory, the heat produced will mobilise the lighter fractions as a combustion front moves steadily through the formation, with the heavier tars burning. In practice, however, it is almost impossible to control the progress since the gases tend to rise while the water present sinks, resulting in an uneven combustion front.
A fourth approach is microbial enhanced oil recovery (MEOR). This entails the use of micro-organisms such as bacteria to dislodge the oil, and a number of systems have been proposed. In the case of unconsolidated measures, such as oil shales, the oil bearing rock may be pumped as an aqueous slurry to surface settling tanks or reservoirs where it is subjected to aerobic bacteria, as disclosed in U.S. Pat. No. 2,907,389. The availability of oxygen allows the bacteria to multiply, using the oil as a carbon source. In so doing, the bacteria produce surfactants which act to free the oil as droplets. The droplets of oil are less dense than water and so float to the surface. The oil is then removed. Unfortunately, the system cannot conveniently be applied to consolidated rock formations, particularly when they are undersea.
In situ MEOR methods generally fall into two categories, aerobic bacteria systems, as described typically in U.S. Pat. No. 3,332,487 and anaerobic bacteria systems as described in WO 89/10463.
The very existence of oil in a formation means that there cannot be present any anaerobic bacteria which will feed on oil under the prevailing conditions. Thus, in anaerobic bacteria systems, it has been assumed that a source of carbon or “food” must be supplied. However, under these circumstances, the bacteria selected (either deliberately or naturally) will be those most suited under the prevailing conditions to the consumption of the particular food employed. They will not be specifically adapted to have an effect on oil and therefore their action on oil will be as it were a by-product and these anaerobic systems therefore tend to be very slow in achieving the desired liberation of oil.
The absence of any oxygen in oil bearing formations means that if an aerobic system is to be used, then oxygen must be supplied. However, when aerobic bacteria are used and oxygen (or air, containing oxygen) is injected into the formation, the situation is far from satisfactory. Firstly, there is an immediate separation into a gaseous and an aqueous phase, which makes control of the system very difficult and in practice, limits the system to a batch-type operation. Secondly, a great deal of heat is generated, which, in view of the oxygen-rich gaseous phase and the readily available combustible material, presents a considerable risk of explosion. A cooling medium must therefore also be employed.
One way of addressing this is described in WO 92/13172. This reference describes a method of microbial enhanced oil recovery for recovering oil from an oil-bearing second rock formation, the formation including an inlet at a first location and an outlet at a second location, the method comprising: injecting water containing a source of oxygen, capable of yielding at least 5 mg/l free oxygen, into the formation at the first location spaced apart from the second location, allowing micro-organisms, which are either already present in the formation or which are introduced simultaneously with the oxygen containing injection water to multiply using the oil as their main carbon source and the oxygen from the injection water as their main oxygen source thereby establishing a biomass layer which acts to dissociate the oil from the rock formation, the dissociated oil then being removed via the outlet by the injection water.
In this system, on the far side of the injection well, the oxygen becomes the growth limiting factor due to the consumption of oxygen by the micro-organisms. The rate of growth of micro-organisms is of course dependent on the available oxygen. In this system maximum growth is desired and therefore it is desirable to maintain a high oxygen concentration in the injection water (and clearly also in advancing biomass layer). Field trials in Austria in recent years have shown that by the use of this system, it has been possible to obtain a threefold increase in oil production.
However it has been found that this system is effective over only a relatively short distance from the point of oxygen injection. Furthermore, it would be desirable to avoid the use of injected oxygen in order to avoid problems of corrosion. This is particularly acute in situations where the equipment susceptible of corrosion is remote and/or difficult and costly to replace.
It is known that sulphate reducing bacteria (SRB), nitrate reducing bacteria (NRB), iron-reducing bacteria (IRB) and acetogenic bacteria can grow anaerobically on oil if they have the necessary inorganic nutrients. Laboratory studies have shown that oxygen can be substituted by certain growth factors in order to achieve a reduction in residual oil concentration.
The main obstacle in this process is that the growth factors, as with Oxygen, are consumed near the injection point which limits the range of effectiveness of the process.